A cemented carbide containing tungsten carbide (WC) as a main component, containing cobalt (Co), nickel (Ni), or the like as a binder metal, and having a carbide of Ti or Ta added thereto for improvement of performance is excellent in hardness and wear resistance, and is therefore widely used in various fields as a cutting tool, a wear-resisting tool, a mining tool, or the like. At present, there are a large amount of scraps of such tools having been used in the various fields. Hence, recycling thereof has been drawing attention.
Among them, tungsten, which is a scarce resource, is contained in each of the above-described tools by approximately 80 mass %. Accordingly, various methods have been proposed to recover tungsten from such a tool scrap. For example, the scrap is smelted with sodium nitrate to obtain sodium tungstate (Na2WO4) as a water soluble salt, which is then converted into ammonium tungstate ((NH4)2WO4) to recover tungsten carbide in the end.
Ammonium tungstate can be highly purified as ammonium paratungstate (APT) by crystallizing an aqueous solution thereof. By calcinating, reducing, and carbonizing the crystal thus obtained, tungsten carbide can be readily obtained. Hence, ammonium tungstate is an important substance in the above-described recycling process.
An exemplary, proposed method for producing such ammonium tungstate is to produce an ammonium tungstate aqueous solution by bringing a sodium tungstate aqueous solution into contact with a Cl type strong base anion exchange resin, and thereafter eluting using ammonium chloride and an ammonia-containing aqueous solution (Japanese Patent Laying-Open No. 2008-150251 (Patent Literature 1)).
Alternatively, for example, Japanese National Patent Publication No. 2000-514030 (Patent Literature 2) disclose a method of obtaining a high-purity tungstate solution by treating, with an acid, an alkaline tungstate solution contaminated by vanadium, molybdenum, and the like, so as to adjust pH thereof to 7 to 10 to produce a precipitate containing vanadium; filtering it; separating therefrom impurities other than the molybdenum ions, by means of an ion exchanger with a weak or moderate basicity; filtering it; treating this filtrate using a sulfide to produce thiomolybdate; and separating this thiomolybdate using a sulfide type ion exchanger.
Furthermore, for example, Chinese Examined Patent Publication No. 1016158 (Patent Literature 3) discloses a method of obtaining a high-purity tungstate solution by sulfurating MoO42− ions in the tungstate solution to produce MoS42− ions; and separating the MoS42− ions therefrom by selectively adsorbing them to a strong base anion exchange resin.